640 Reversing by Link Motion. 4 K t n N ,. which the engine is to be turned. The slide valve K once opened, G may be dropped, crank c catches up the sheave A by the stop D, F find its way to the valve rod pin, and the gear is once more automatic. The engine may be stopped by lifting the gab. Reversing by Link Motion.—If two fixed eccentrics be placed on the shafts, one for forward and one for backward move- ment, it can be arranged to put either eccentric in gear as required, the other remaining inactive. The gear for this purpose is known as link motion, and, though more complicated than loose-eccentric gear, is more easily manipulated, and is absolutely certain in action whatever the position of the crank. In Stephen- son's Link Motion^ Fig. 637, the eccentric rods A B are connected to either end of a link c, curved to a radius from D. The valve spindle F supports a die E capable of vertical movement relatively to link c, such movement being controlled by the lifting link G. At present the radius link is in * mid gear/ and any * plus ' move- ment of one eccentric rod would be met by a ' minus' movement of the other rod. If these movements were equal, the valve would not travel at all; but, as the sheaves are not placed directly opposite on the shaft, the plus and minus displacements do not balance, and the valve opens-to lead.* If the reversing rod H be moved to the right, the rocking link G will lift the radius link until rod B is nearly level with the valve spindle, and the valve then receives almost all the horizontal movement of the B, while A'S motion is all but inoperative on the valve. The eccentric B is then in ' full' gear. If H be moved to the left, the A rod is put in gear and B is practically inoperative, j is termed the weigh-bar shaft, and H is coupled to a hand lever on the driver's platform.! In Gooctts Link Motion^ Fig. 638, the eccentric rods A and B always vibrate at the same height, and radius link c rocks from a fixed point G. But the valve rod is in two parts, one of which, K, the intermediate valve rod, being lifted or lowered, changes also the position of the die E. In the figure, K is shewn in direct connection with the rod A, while B'S vibration has no effect on the valve. When K is at its lowest position, rod B is in gear and A is noperative; link c has it curves struck from r>. It should also * Larger lead than that ia full gear. t In large marine engines it is usual to reverse by steam power.